SERVER-9983 Do not needlessly lock when looking up privileges for the __system@local user.

Uncorrected, this can cause replica set heartbeats to stall behind operations
that hold the read lock for a long time.
SERVER-9983 Test verifying that internal user privileges do not mask those of similarly named users.
SERVER-7080 Add code to emulate backtrace()

backtrace() fills a buffer provided by the caller with addresses
constituting a stack trace.  This emulation allows the display of
addresses in Solaris 10, which can be used with addr2line to see
code locations.